(1) Field of the Invention
The present invention relates to an image forming apparatus, and in particular to technology for improving productivity by stably driving a stepping motor in a wider speed range.
(2) Description of the Related Art
In the field of image forming apparatuses, when a toner image or a recording sheet is transported, an accurate positional control and a timing control are required. For such a control, a stepping motor is widely used.
The driving methods for a stepping motor include a constant-voltage control method and a constant-current control method. In the constant-voltage control method, if the rotation speed of a stepping motor is low, large current flows in an excitation coil, causing overheat and damage. On the other hand, in the constant-current control method, a current flow is controlled at a constant amount. In this way, there is no risk for a large current flow. For this reason, the constant-current control method is employed in a low-speed range (see Japanese Patent Application Publication No. H11-122990).
Here, excitation coils exhibit variations in coil properties such as inductance. Accordingly, even if the constant-current control method is employed, variations in coil properties lead to variations in torque between phases. The variations in torque are pronounced in a middle-speed range, causing the stepping motor to vibrate (hereinafter this phenomenon is referred to as “middle-range vibration phenomenon”). When the middle-range vibration phenomenon occurs, the torque of the stepping motor is decreased. This results in a malfunction of the image forming apparatus, such as a loss of synchronization or stopping of the motor.
To resolve the middle-range vibration phenomenon as described above, the following technique is proposed, for example. That is, for each phase, torque is calculated from the current area in each period of the waveform of current supplied to the excitation coil. If variations in torque are large between the phases, further acceleration of the stepping motor is prevented (see Japanese Patent Application Publication No. 2010-4592).
In this way, even if coil properties vary between the phases of the stepping motor, the operation of the stepping motor is stopped in the speed range in which the middle-range vibration phenomenon occurs. Accordingly, a malfunction of the image forming apparatus due to a loss of synchronization or stopping of the stepping motor is prevented.
However, there is a demand for the stepping motor to increase speed so as to improve the productivity of the image forming apparatus, and to operate over a wide range of system speeds. To meet such demands, the stepping motor is required to stably operate in a wide speed range from a low-speed range to a high-speed range.
On the other hand, according to the conventional technology as described above, the stepping motor cannot be used in the middle- to high-speed range due to the vibration phenomenon that occurs in the middle-speed range. For this reason, the conventional approach to resolve the middle-range vibration phenomenon has a limitation in terms of increasing speed and increasing the speed range.